To cause infection, membrane-enveloped viruses such as human immunodeficiency virus type 1 (HIV-1) must first bind receptors at the cell surface and subsequently fuse their own membrane with that of the cell. This fusion process releases the viral genetic material into the cytoplasm and initiates infection. Entry of HIV-1 into cells has long been known to require virus binding to the cell surface molecule CD4 (Maddon, et al., 1986; Sattentau and Weiss, 1988), and recent studies have defined chemokine receptor molecules, CXCR4 (Feng, et al., 1996; Berson, et al., 1996), CCR5 (Alkhatib, et al., 1996; Choe, et al., 1996; Deng, et al., 1996; Doranz, et al., 1996; Dragic, et al., 1996), and CCR3 (Choe, et al., 1996; Doranz, et al., 1996) as co-receptors required for entry. HIV entry occurs in stages: initial binding of the viral envelope protein (designated gp120/41) to CD4 is followed by conformational changes that allow binding to coreceptor and subsequent coalescence of the viral and cellular membranes (Lapham, et al., 1996; Trkola, et al., 1996; Wu, et al., 1996). HIV-infected cells express gp120/41 on the cell surface during infection and prior to its incorporation into budding HIV-1 particles.
Recent studies have shown that vescisular stomatitis virus (VSV) has potential as a high level expression vector capable of incorporating foreign proteins into the viral envelope (Schnell, et al., 1996a; Schnell, et al., 1996b). VSV causes an extremely rapid cytopathic infection in most animal cells including human T cells in culture, but is normally non-pathogenic in humans (for review see Wagner and Rose, 1996). Within two to three hours after infection, VSV blocks host cell protein synthesis and within eight hours produces very large quantities of progeny virus particles that bud from the cell surface prior to cell lysis. VSV has a non-segmented, negative-strand RNA genome that is copied in the cytoplasm of infected cells by the viral RNA polymerase to generate five mRNAs encoding the five structural proteins. Only one of these proteins, a glycoprotein designated G, is present in the viral membrane and is responsible for the very broad host range of VSV. G protein recognizes cell surfaces and catalyzes fusion of the viral membrane with cellular membranes (Florkiewicz and Rose, 1984). Foreign membrane proteins such as CD4 and other viral membrane proteins can be expressed at very high levels from the genome of recombinant VSVs and these molecules are then incorporated at high levels into the viral membrane along with G protein (Schnell, et al., 1996b).